Fourier transform infrared (“FTIR”) spectrometers are well known in the art. Michelson interferometers function by splitting a beam of electromagnetic radiation into two separate beams via a beam splitter. Each beam travels along its own path, e.g. a reference path of fixed length and a measurement path of variable length. A reflecting element, such as a retroreflector, is placed in the path of each beam and returns them both to the beam splitter. The beams are there recombined into a single exit beam. The variable path length causes the combined exit beam to be amplitude modulated due to interference between the fixed and variable length beams. By analyzing the exit beam, the spectrum or intensity of the input radiation can, after suitable calibration, be derived as a function of frequency.
When the above interferometer is employed in a FTIR spectrometer, the exit beam is focused upon a detector. If a sample is placed such that the modulated beam passes through it prior to impinging upon the detector, the analysis performed can determine the absorption spectrum of the sample. The sample may also be placed otherwise in the arrangement to obtain other characteristics.
Where the path length through the interferometer is varied by moving a retroreflecting element along the axis of the beam, the maximum resolution attainable with the instrument is proportional to the maximum path difference that can be produced. Because Michelson interferometers rely upon the interference from recombination of the two beams, a quality factor of such a device is the degree to which the optical elements remain aligned during path-length variation. Thus, translational displacement of the mirror must be extremely accurate. That is, the mirror must in most cases remain aligned to within a small fraction of the wavelength of incident light, over several centimeters of translation. Any deviation from pure translation may cause slight tilting of a plane mirror, leading to distortion in the detected beam. Substitution of cube-corner and cats-eye retroreflectors for plane mirrors can essentially eliminate such tilting distortion problems; but with certain inherent drawbacks.
Precision bearings may be used to maintain alignment. In addition, monitoring and controlling alignment with analysis of feedback and subsequent repositioning has been utilized to maintain mirror alignment. Systems relying on either such solution are difficult to design, relatively large, expensive and present maintenance issues.
Other efforts have been made to develop interferometers that do not require precision bearings or control systems. Tiltable assemblies consisting of a pair of parallel, confronting mirrors have been suggested as replacements to the longitudinally displaced retroreflector. U.S. Pat. No. 4,915,502, issued on Apr. 10, 1990, teaches a twin-arm interferometer spectrometer having a tiltable assembly by which the optical path lengths of the two beams are varied simultaneously. A much smaller rotation, relative to retroreflectors, of the paired mirrors results in the path difference. This design reduces sensitivity to linear movement of the optical element; moreover, rotating bearings are generally easier and less expensive to produce than are longitudinal or linear ones.
U.S. Pat. No. 4,383,762, issued on May 17, 1983 and provides a two-beam interferometer for FTIR spectroscopy in which a pendulum arm holds moving cube corner retroreflectors. The movement, i.e. arcuate oscillation, results in accurate changes in path-length produced in a smooth motion. The retroreflectors render the system unaffected by the tilt and avoids the disadvantages for FTIR spectroscopy that are inherent in the deviation from strict linearity from the pendulous motion.
So-called “porch swing” mounting arrangements are also known in the art. Here, structural elements are supported at four pivot points and form a parallelogram by which a mirror undergoes pure translation along an axis. The extremely high machining tolerances required of such an arrangement and related issue of assembling same, result in high costs of both manufacture and maintenance. In addition, such pure translation flexure mounts are not typically useful for the relatively large displacements necessary for high resolution applications. The need for greater displacement can be achieved, but primarily through great cost of highly engineered precision instrumentation.
Over and above the issues raised above, the mirror-supporting structure must be isolated to the greatest possible degree from extraneous forces which would tend to produce distortions of the structure. Such forces and resultant distortions introduce inaccuracies into the optical measurements. The forces may arise from vibrational effects from the environment and can be rotational or translational in nature. A similarly pervasive issue concerns thermal and mechanical forces. Needless to say, considerations of weight, size, facility of use, efficiency, manufacturing cost and feasibility are also of primary importance.
Accordingly, it would be desirable to provide an optical assembly comprising a flexure mount with pure translation over a sufficiently large displacement at a reasonable cost of manufacture and maintenance. It is also desirable that the optical assembly be isolated from extraneous forces tending to produce optical distortions.